Low wattage fluorescent lamp having improved phosphor layer

A low pressure mercury vapor discharge lamp comprising a light-transmissive glass envelope having an inner surface, means for providing a discharge, an ultraviolet reflecting barrier layer comprising alumina particles coated adjacent said inner surface of said glass envelope, a phosphor layer coated on said barrier layer, and a discharge-sustaining fill of mercury and inert gas sealed inside said envelope, said phosphor layer having a coating weight of 2.0-3.9 mg/cm2, said inert gas comprising a mixture of argon and krypton wherein krypton is 10-40 volume percent of said mixture and the total pressure of said inert gas inside said envelope is 1-4 torr, said lamp having a lumen efficiency of at least 80 lumens/watt.